1. Field of the Invention
The present invention relates to monomers for use in the synthesis of polyurethanes, and more particularly to carboxyl-containing monomers made from a low molecular weight polyol compound and an acid anhydride, and in the presence of 5-500 ppm phosphoric acid.
2. Description of the Related Art
It is well known that polyurethanes are generally manufactured by reacting a polyisocyanate and a polyol. The produced polyurethane may have unique chemical and/or mechanical properties depending on the reacting conditions, as well as other additives such as catalysts, solvents, surfactants, blowing agents, fillers, and the like. The polyols used in manufacturing polyurethanes are typically low molecular weight poly-hydroxyl-containing polymers, such as those containing polyethers, polyesters, polyacrylics, polycarbonates, and the like. These polyols are generally provided with at least two hydroxyl groups so that they can be easily incorporated into a lengthening polymer in an ordered fashion.
Due to environmental and toxicity concerns, water-based polyurethanes and aqueous dispersions of polyurethanes are becoming the materials of choice for many applications, including aqueous applications. However, in many instances, the components of the polyurethane product are not easily soluble in water. To overcome this problem, it is known to introduce ionizable groups into the monomers prior to their condensation into the final polyurethane polymer. These ionizable groups are thought to aid in the solubilization of the polymer and thus produce a uniform aqueous dispersion of the final polyurethane mixture.
The reaction of succinic anhydride with triols or tetrols has been described generally in U.S. Pat. No. 5,863,980 to Kuen-Bae Choi. According to the disclosure, acid groups are introduced in the main chain in the absence of catalysts, including acid catalysts. In order to perform reaction between succinic anhydride and polyol, high reaction temperature was used and resulted in highly viscous polyols. When such highly viscous polyol was reacted with diisocyanate, a viscous prepolymer was obtained.
U.S. Pat. No. 4,207,2267 to Wulf von Bovin discloses a process for preparation of stable suspensions of inorganic fillers in poly-hydroxyl compounds by grafting olefinically unsaturated carboxylic acid onto polyol. As an example, acrylic acid and peroxide type initiators are used for this process.
U.S. Pat. No. 4,250,077 to Wulf von Bovin et al. discloses a suspension which is stable and contains inorganic filler and graft polymer which was produced by free radical polymerization of olefinically unsaturated carboxylic acid.
U.S. Pat. No. 4,460,738 to Frentzel et al. discloses a process for grafting carboxyl groups to mono and polyether polyols by reacting maleic acid, fumaric acid, itaconic acid or their mixtures with polyether polyols in presence of peroxy-type free radical initiator.
U.S. Pat. No. 4,521,615 to Frentzel et al. discloses a process for grafting carboxyl groups to mono and polyether polyol by reacting maleic acid, fumaric acid or their mixtures with short chain polyether polyols in presence of peroxy-type free radical initiator.
U.S. Pat. No. 5,990,250 to Housel et al. discloses a process for incorporating carboxyl groups into main polyester chain by reacting polyether or polyester polyol with aliphatic dianhydride.
U.S. Pat. Nos. 5,242,954 and 5,250,582 to Hire et al. disclose a process for making cellular and microcellular polyurethane foams using a carboxylic acid-grafted polyol.
A common result of introduction of a carboxyl group into the polyol component is undesirable side reactions between the carboxyl group and nearby hydroxyl groups. The side reactions markedly increases viscosity of the monomer mixture, and provides fewer usable monomers for incorporation into the final aqueous urethane dispersion. In addition, the reacted carboxyl group results in reduced hydrophilicity of the final urethane dispersion.
Accordingly, there is a need in the art for water-soluble monomers to be incorporated into a urethane dispersion that possesses low viscosity, and does not undergo undesirable side reactions. The present invention is believed to be an answer to that need.
In one aspect, the present invention is directed to carboxyl-containing monomers for use in preparing a polyurethane polymer, the carboxyl-containing monomer being the reaction product of a low molecular weight polyol compound and an acid anhydride in the presence of 5-500 ppm phosphoric acid, the carboxyl-containing monomer having a viscosity in the range of about 3,000-100,000 centipoise, and having free oligomer content in the range of about 2-30 mg KOH/g.
In another aspect, the present invention is directed to a method of preparing a carboxyl-containing monomer for use in preparation of a polyurethane polymer, comprising the step of combining a low molecular weight polyol compound and an acid anhydride in the presence of 5-500 ppm phosphoric acid to produce the carboxyl-containing monomer, the carboxyl-containing monomer having a viscosity in the range of about 3,000 to about 100,000 cps and having a free oligomer content in the range of about 2 to about 30 mg KOH/g.
In another aspect, the present invention is directed to a prepolymer for use in preparing a polurethane polymer, the prepolymer being the reaction product of (1) the carboxyl-containing monomer described above, and (2) a polyisocyanate compound, the prepolymer having a viscosity in the range of about 3,000 to about 100,000 cps.
In another aspect, the present invention is directed to a method of preparing a prepolymer for use in preparation of a polyurethane polymer, comprising the step of combining the prepolymer described above with a polyisocyanate compound to produce the prepolymer, the prepolymer having a viscosity in the range of about 3,000 to about 100,000 cps.
In another aspect, the present invention is directed to a water-borne polyurethane polymer, the water-borne polyurethane polymer being the reaction product of (1) the prepolymer of described above, and (2) an amine compound.
These and other aspects will become apparent upon reading the following detailed description of the invention.
It has been surprisingly found, in accordance with the present invention, that a carboxyl-containing monomer being the reaction product of a low molecular weight polyol compound and an acid anhydride, and made in the presence of phosphoric acid, results in a chemical monomer that is very beneficial for making waterborne polyurethane dispersions. The present inventors have unexpectedly discovered that phosphoric acid is very efficient in catalyzing polyol-anhydride addition reactions with anhydride ring opening mechanism, while exhibiting little or no acceleration of acid and polyol condensation side reactions.
In contrast to the prior art where the hydroxyl groups of polyol are reacted with polyisocyanate to produce polyurethane prepolymer, the monomer made according to the present invention possesses an polar center (by virtue of the grafted carboxyl function) that is thought to stabilize and solvate waterborne polyurethane emulsions when polyurethane prepolymer is dispersed in a water or alkaline-water solution. In addition, the presence of phosphoric acid in the reaction medium is thought to maintain the grafted carboxyl group in a protonated state and thus prevent it from participating in undesirable side reactions which lead to a high viscosity product that has limited usefulness.
The present invention provides a process for xe2x80x9cgraftingxe2x80x9d carboxyl groups to polyol monomers, and the low-viscosity carboxyl-containing polyol monomers made by the process. The term xe2x80x9cgraftingxe2x80x9d refers to addition of one molecule onto another molecule by means of a chemical reaction. The process of the present invention generally consists of reacting a polyol monomer, preferably containing three hydroxyl groups per molecule, with an organic acid anhydride under conditions such that an organic acid group is grafted to the polyol monomer. Because the polyol monomer is preferably selected to have three hydroxyl groups per molecule and is reacted with only one molecule of anhydride, the resulting carboxyl-containing monomer possesses two free hydroxyl groups per molecule and one carboxyl group grafted to the polyol. As indicated above, the two free hydroxyl groups of the carboxyl-containing monomer are used in subsequent reactions that form the ultimate polyurethane, while the carboxyl group aids in hydration of the polyurethane dispersion and prevents generation of highly viscous, unwanted side reactions and undesirable by-products.
As defined herein, the term xe2x80x9cpolyolxe2x80x9d refers to compounds having between two and four free hydroxyl (xe2x80x94OH) groups per molecule, and preferably three hydroxyl groups. As defined herein, the phrase xe2x80x9clow molecular weight polyolxe2x80x9d refers to those polyols having a molecular weight less than 8,000, more preferably less than 2,000, and most preferably less than 500. The phrase xe2x80x9ccarboxyl-containing monomerxe2x80x9d refers to a polyol having a carboxyl group added to one of the hydroxyl groups of the polyol.
As indicated above, in one aspect, the present invention is directed to a carboxyl-containing monomer for use in preparing a polyurethane polymer. The carboxyl-containing monomer is the reaction product of a low molecular weight polyol compound and an acid anhydride, and the resulting carboxyl-containing monomer has a viscosity in the range of 3,000-100,000 centipoise (cps) and has oligomer content in the range of 2-30 mg KOH/g. Each of these components are discussed in more detail below.
Examples of polyols that are useful in the present invention include low molecular weight polyols having from two to four hydroxyl groups. Preferably, the polyol contains three free hydroxyl groups (hereinafter termed xe2x80x9ctriolxe2x80x9d). Triols suitable for use in the present invention are generally based on the structure of glycerol, trimethylolpropane, trimethylolethane, and the like. Preferred triols include Poly-G 76-635 (a polyether triol of molecular weight 265, available from Arch Chemicals, Inc..) and Poly-G 35-610 (a polyether triol of molecular weight 275), and their mixtures with trimethylolpropane or pure trimethylolpropane. Alternatively, polyalkylene polyether polyols produced by the poly-addition of any of the mentioned above triols and an alkylene oxide such as ethylene oxide, propylene oxide, butylene oxide, epoxybutene, and the like, may also be used. These triols usually have molecular weight from less than 100 to about 6000.
Suitable acid anhydrides used in the present invention include any acid anhydride that results in the addition of a carboxyl group to the monomer molecule. Useful acid anhydrides include maleic anhydride, phthalic anhydride, succinic anhydride, glutaric anhydride, and mixtures thereof. A preferred acid anhydride is succinic anhydride.
The preparation of the carboxyl-containing monomer is generally accomplished by reacting the low molecular weight polyol compound with an acid anhydride in the presence of phosphoric acid. Generally, the polyol is heated with anhydride to about 80-105xc2x0 C. in presence of about 5-500 ppm of orthophosphoric acid (H3PO4), more preferably from about 100-400 ppm orthophosphoric acid, and most preferably from about 250-350 ppm orthophosphoric acid. According to the present invention, addition of 5-500 ppm of orthophosphoric acid catalyzes the selective reaction between polyol and anhydride, and an acid group is xe2x80x9cgraftedxe2x80x9d to the polyol molecule. Preferably, a triol is selected that has three hydroxyl groups per molecule, and each triol molecule is reacted with one molecule of anhydride to generate a product that has two hydroxyl groups per molecule and one carboxyl group grafted to the polyol.
The reaction is monitored by acid number, and as soon as theoretical acid number has been reached, the reaction is stopped. In order to dry the reaction product, a solvent capable of forming an azeotropic mixture with water is added. Later, the azeotropic mixture of solvent and water is removed by vacuum distillation.
Preferably, the carboxyl-containing monomers are liquid at room temperature because liquids are more easy to handle as compared to solids. A useful range of viscosities for the carboxyl-containing monomers is generally less than 100,000 cps at 25xc2x0 C. Preferably, the viscosity of the carboxyl-containing monomers is from about 3,000 to about 100,000 cps, more preferably from about 3,000 to about 50,000 cps, and most preferably from about 3,000 to about 20,000 cps.
The viscosity of prepolymers made with carboxyl containing monomers is preferably less than 100,000 centipoise (cps) at 25xc2x0 C. in order to obtain good water borne dispersions. A preferred range of viscosity is from about 3,000 to about 100,000 cps, more preferably from about 3,000 to about 50,000 cps, and most preferably from about 3,000 to about 20,000 cps. If viscosity of prepolymer is more than 100,000 cps, the prepolymer usually is too thick for high speed mixing and no good waterborne dispersion can be obtained.
In order to increase the shelf life of prepolymer products made from the carboxyl-containing monomers, it is desirable that the carboxyl-containing monomers made as described above contain minimal amounts of oligomers. As defined herein, oligomers are molecules which result from the reaction of the grafted carboxyl function with another hydroxyl function, which can lead to oligomerization of the monomer products. Oligomers are undesirable due to their propensity to cause increased viscosity of the monomer product.
It has been found that the presence of oligomers above about 30 mg KOH/g (as analyzed below) results in undesirable gelling of the prepolymer product. Preferably, the carboxyl-containing monomers have less than 30 mg KOH/g oligomers, preferably between 2 and 30 mg KOH/g oligomers, more preferably between 2 and 20 mg KOH/g oligomers, and most preferably between about 2 and 15 mg KOH/g oligomers. Oligomer content in the carboxyl-containing monomer can be measured by calculating the difference between theoretical acid number and acid number determined by chemical analysis as known in the art.
Briefly, acid number is determined using 1-2 grams of sample. 100 ml of isopropyl alcohol and 50 ml water is added to the sample, and stirred until the sample is completely dissolved. Approximately 15 drops of 1% phenolphtalein solution is added, and the sample solution is titrated with 0.5 N potassium hydroxide (or 0.5 N sodium hydroxide) until a light pink color appears. Oligomer content in the carboxyl-containing monomer can be measured by calculating the difference between theoretical acid number and acid number determined by chemical analysis (expressed as mg KOH/g sample). This difference in mg KOH/g is then correlated to oligomeric ester units per gram of monomer.
As indicated above, the carboxyl-containing monomers prepared above may be used in the production of a xe2x80x9cprepolymerxe2x80x9d. In general, the prepolymer is made by combining the carboxyl-containing monomers prepared above with a polyisocyanate compound. Organic polyisocyanates useful as reactants in the production of the prepolymer include any aromatic, cycloaliphatic and aliphatic diisocyanates and higher polyisocyanates. Diisocyanates are the preferred class of polyisocyanates. Suitable aliphatic diisocyanates include hexamethylene diisocyanate, 4,4xe2x80x2-dicyclohexylmethane diisocyanate; isophorone diisocyanate;1,4xe2x80x2-tetramethylene diisocyanate; and 1,10-decamethylene disiocyanate and 1,12-dodecamethylene diisocyanate. Suitable aromatic diisocyanates include tolulene-2,4- or 2,6-diisocyanate; 1,5-naphthalene diisocyanates; 4-methoxy-1,3-phenylene diisocyanate; 4-chloro-1,3-phenylene diisocyanate; 2,4xe2x80x2-diisocyanatodiphenyl ether; 5,6-dimethyl-1,3-phenylate diisocyanate; 2,4-diemthyl-1,3-phenylene diisocyanate; 4,4xe2x80x2diisocyanatodiphenylether; benzidene diisocyanate, 4,4xe2x80x2-diisocyanataodibenzyl; methylene-bis(4-phenylisocyanate); and 1,3-phenylene diisocyanate. Particularly useful polyisocyanates for use in preparing the polyurethane prepolymers include toluene diisocyanate, 4,4xe2x80x2-diphenylmethane diisocyanate, 2,4xe2x80x2-diphenylmethane diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, 4,4xe2x80x2-dicyclohexylmethane diisocyanate, 1,12-dodecanediisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, xylylene diisocyanate, tetramethyl-xylylene diisocyanate and other polyisocyanates such as polymethylene polyphenyl isocyanate and isocyanate prepolymers having at least two isocyanate groups which are produced by reacting an isocyanate with a polyhydroxyl compound such as polyoxyalkylene polyol or polyester polyol or mixtures thereof.
The reaction in which hydroxyl groups are reacted with isocyanate groups and polyurethane prepolymer is produced is usually performed at 50-100xc2x0 C. for 1-5 hours under an inert atmosphere such as nitrogen gas and at atmospheric pressure. Preferably the reaction is performed at 70-90xc2x0 C. for 2-3 hours.
The ratio of isocyanate to carboxyl-containing polyol is such as to have the desired amount of grafted carboxyl groups per molecule of polyurethane prepolymer. Usually, the carboxyl-containing monomer is added to result in an acid number for the prepolymer of 10-30 mg KOH/g. The preferred procedure for producing the prepolymer is to react the selected polyisocyanate with regular polyether or polyester polyol for 1-2 hours at 80-90xc2x0 C., and then add carboxyl-containing monomers until the theoretical isocyanate group content has been reached. If desired, catalysts such as dibutyltin dilaurate, stannous octoate, or amine-type catalysts like triethylamine or triethylene diamine, may be used to assist prepolymer formation. The prepolymer composition may also include solvents such as methylethylketone, methylpyrrolidone, and the like
Because carboxyl groups are grafted to the polyol molecule, the resulting main polyurethane chain is linear with carboxyl groups as side pendants. This structure is ideal for obtaining good water-borne dispersions. The chemical structure of an exemplary prepolymer made from 1 mole of 1000 molecular weight propylene oxide based diol (Poly-G 20-112 from Arch Chemicals, Inc., Norwalk, Conn.), three moles of 4,4xe2x80x2dicyclohexylmethane diisocyanate, and one mole of trimethylolpropane with succinic anhydride grafted to a side chain is as follows: 
This prepolymer is easy to disperse in water by converting side pendant carboxyl groups into salt groups and then reacting free NCO groups with diamine to obtain a high molecular weight urethane dispersion in water. Because, according to the present invention, there is one carboxyl group grafted to each triol molecule, the resulting prepolymer has low viscosity, low oligomer content, and is very easy to disperse in water. The dispersion process proceeds easily and water-borne dispersions may be prepared without the use of high shear/high speed mixers.
The prepolymer described above, may be combined with an amine compound to extend the prepolymer and further dispers the polymer in water. Suitable amines for dispersing prepolymer in water and chain extending the prepolymer include triethylamine, tripropylamine, ethylene diamine, n-butylamine, diethylamine, trimethylamine, monoethanol amine, dimethylethanolamine, aminoalcohols, hydrazine, hexamethylene diamine, isophorone diamine, cyclohexane diamine, dimethylcyclohexylamine, tris(3-aminopropyl)amine, 2-methylpentamethylenediamine, 1,12-dodecanediamine and combinations thereof.
The chain extension reaction occurs when free isocyanate groups of water dispersed prepolymer react with amino groups and is described in the art. The reaction between isocyanate groups and amine groups is very fast and chain extension step can be carried out in water.
It may be desirable to add other conventional additives such as blowing agents, thickening agents, pH adjusters, monoisocyanates and the like to the composition of the invention. Furthermore, fillers, plasticizers, pigments, and the like may be utilized as desired. It may be also desirable to add other polyurethane prepolymers made from modified or unmodified polyether polyols or polyester polyols or the like.
The following examples are intended to illustrate, but in no way limit the scope of the present invention. All parts and percentages are by weight and all temperatures are in degrees Celsius unless explicitly stated otherwise.